Simple Pendelum - Help with Quadratic Regression on Excel?

1.
I am trying to determine the value of g based on a simple pendulum.
I have graphed a scatter plot with the x axis as L and the y axis as P. I have 3 different series for different lengths of pendulum. When I try to find a line of best fit I get something like cx^2 + dx + e and this trend line is only for one series.

2 Parts - Is there any way to get the trend line to be the line of best fit for all my different series (different lengths)
- Is there any way to make my quadratic regression approximate only in terms of ax^2.

2.
The equation, for a small angle approximation works out to L=(G/((2pi)^2)) P^2. Where L is the length of the Pendulum in meters, G is Gravity and P is the period of the pendulum.

3. excel graph - the regression gives a wildly incorrect value of G. Attempt: Determined the formula and how to get the solution

One series consists of a set of measurements of the period, all with the same length? If so, why would you fit a quadratic to that? Why isn't it just a matter of taking the mean? Or, throw all the data into one chart, using an XY plot?